In many situations, apparatuses must operate in potentially hazardous conditions, such as where a fuel mixture may be ignited by uncontrolled operating or environmental conditions. For example, vehicles, including aerospace vehicles, typically operate with a fuel that must be maintained in a safe condition during storage and use. The ignition hazard should be minimized even when the vehicle is subject to uncontrolled events such as an accident, electrical malfunction, a lightning strike, or static electrical discharge. Other applications requiring ignition hazard consideration include fuel transport, fuel storage, mining operations, chemical processing, metal fabrication, power plant construction and operation, and operations which involve combustible particulate such as sawdust, metal, flour, and grain.
Design of apparatuses exposed to ignition hazards typically involves reducing the likelihood of ignition, containing the ignition hazard, and/or withstanding the ignition hazard. Test systems may facilitate or verify the design of a component by simulating or applying ignition hazard precursors such as heating, a simulated lightning strike, or other electromagnetic effects (e.g., arcing, electrostatic discharge, heating, and/or hot particle ejection).
In the aerospace industry, the Federal Aviation Administration (FAA) requires ignition source tests for components potentially exposed to fuel-vapor environments (specified in SAE ARP 5416A (SAE Aerospace)). One test method is the photographic method and another method is the ignitable mixture (flammable gas) test method. The photographic method subjects a test article to an ignition hazard precursor (e.g., simulated lightning strike) in a darkened chamber to observe the light produced by arcs (if any) emitted by the test article. The photographic method is suitable for arcs, which are relatively quick and bright, but not well suited for hot particles, other heat sources, or ignition hazards that impart energy more slowly or which do not have corresponding light emission.
The ignitable mixture (flammable gas) test method subjects the test article to the ignition hazard precursor in a flammable atmosphere within a combustion chamber (which may be referred to as a combustion vessel or a bomb). If the test article produces an ignition hazard, the flammable atmosphere explosively ignites in the combustion chamber. The contained explosion may be detected by various techniques (e.g., by detecting the pressure change, the light, the heat, and/or the sound of the explosion). The ignitable mixture test method has the advantage of being more realistic than the photographic method, in that the flammable atmosphere reacts in the same manner (thermal reaction) as does the atmosphere that may contact the test article in actual use. The primary disadvantage of the ignitable mixture test method is the use of the flammable atmosphere, which requires care in the preparation and fabrication of the combustion chamber, and care in handling the flammable gas. Some test articles may be quite large (e.g., components or the entirety of a wing fuel tank of an aircraft) and have consequent greater demands for the safe operation of the combustion chamber and the safe handling of the flammable atmosphere. Additionally, because an ignition hazard causes the flammable atmosphere to be consumed, the ignitable mixture (flammable gas) test method produces merely a pass-fail result.